1. Field of the Invention
This invention relates generally to an all-wheel-drive coupling for dividing torque between primary and secondary drive axles of a motor vehicle.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
It is known for vehicle drive systems to include a vehicle drive apparatus that divides torque between primary and secondary drive axles of a motor vehicle. Generally, such a coupling apparatus will include a transfer case that houses a hydraulically actuated clutch pack for transferring a desired amount of torque to a secondary drive axle of an automotive vehicle. A hydraulic oil pump provides hydraulic pressure to actuate the clutch pack.
In designing an all-wheel-drive apparatus for passenger vehicles and trucks it's beneficial to reduce the cost of the apparatus until the cost is low enough compared to the cost of two-wheel-drive architectures, that the all-wheel-drive apparatus is able to achieve an acceptable degree of market penetration. One of the ways to reduce the cost of an all-wheel-drive apparatus is through systems integration, i.e., by integrating functions that the all-wheel-drive apparatus has in common with other systems in an automotive vehicle. If functions that an all-wheel-drive apparatus has in common with other systems can be integrated in such a way as to support those functions with common hardware instead of duplicate, independent hardware, a lower system cost can be achieved.
For example, the drive system in at least one year model of the Audi A8 includes a vehicle drive apparatus having a hydraulically actuated all-wheel-drive clutch pack that uses an automatic transmission as a source of hydraulic pressure. The automatic transmission has a hydraulic pump for actuating its own hydro-mechanical components, and this source of hydraulic pressure is tapped for use in actuating the all-wheel-drive clutch pack. A hydraulic line extends and provides hydraulic fluid communication between the all-wheel-drive clutch pack and the hydraulic pump of the automatic transmission.
However, there are compatibility problems associated with the use of an automatic transmission hydraulic pump as a pressure source for an all-wheel-drive clutch pack. These compatibility problems must be overcome before the hydraulic pump of an automatic transmission can be used to actuate an all-wheel-drive clutch pack. For example, all-wheel-drive clutch packs typically require a greater magnitude of hydraulic pressure to operate properly. While hydraulic power supplies for actuating all-wheel-drive clutch packs generally output hydraulic pressure in the range of 1000–2000 psi, hydraulic power supplies for automatic transmissions only provide about 50–200 psi of hydraulic pressure.
What is needed is an automotive drive system that integrates a vehicle drive apparatus into a vehicle drive system by allowing the vehicle drive apparatus to share a compatible hydraulic power supply with another drive system component. What is also needed is such a vehicle drive apparatus that is adapted to use such an existing hydraulic power supply to actuate the clutch pack of the apparatus.